In a deregulated telecommunications environment, a problem arises in providing means for telephone service users to access networks in addition to the original PTT telephone network, such as is provided by, for example, British Telecom in the U.K. Such an additional network might be the network of another carrier providing the same range of services as the original PTT network, or a network providing an enhanced or specialized range of services, such as the so-called "Value Added Network". The problem arises because the PTT owns and operates the local network comprising the local exchange and the wire pairs over which telecommunications access to the customer's premises is provided. Typically the problem is solved by allocating to the customer a number within the local numbering plan of the original network for each additional network, to give access to that network through the original network's local exchange. This number is thus used as a prefix to the numbers in the additional network's numbering scheme. The prefix may be generated automatically as a result of a single operation, for example by depressing a key on the customer's telephone equipment. There is, however, a time penalty associated with the signalling of the prefix for outgoing calls, while for incoming calls from the additional network there is the need for this network to signal into the original network's local exchange to complete the switching of the call to the subscriber. There may also be the need for number translation when crossing carrier network boundaries.